Now should one not fulfill these obligations do they have to be re-enrolled? Confess to a priest? Is it a sin to neglect the duties once enrolled?
Also, can the Scapular be taken off for things such as showers and sleep? . . . Now what if one slips in the shower and dies without the Scapular? Or dies at night without it on? Are the promises naught without wearing it?
You don't have to get re-enrolled; one enrolment lasts a lifetime. You don't have to Confess not wearing it or not saying the rosary or Office; it is not a sin to not do these things once enrolled.
The Scapular was originally given by Mary as a full habit, if I have the story right. The small version is for us laypeople; but Mary would not expect nuns and monks to wear their habits in the shower, or to bed, I think; so I would say that is not necessary, but it is up to personal preference. I was told most of this by my priest when he enrolled me when I was a child.
I don't know about the technicalities of dying while in the shower or sleeping, not wearing the scapular; however I do know that the scapular is not essential to Salvation. I.E. as long as you are in the state of grace, there is no reason to fear for your soul if you take your scapular off to sleep or shower . . . or for any other reason, even if you never wear one.
I myself have tried over and over again to force myself to wear the brown scapular, to no avail. The wool, or the laminate, or the cords . . . something always drives me nuts to the point of distracting me almost constantly from the duties of my state in life, and driving me almost to the point of cussing; so I no longer attempt to wear it. However, I am not worried because I frequent the Sacraments and I know I am in the state of grace.
Remember, many hundreds of years of Catholics survived, and many many hundreds of cannonized saints got to heaven, before the scapular was given to us. So I would say, don't worry too much if you dont' shower in it, don't sleep in it, don't fulfill the conditions, or don't wear one at all. It's not essential to salvation.